Scandium is extremely valuable as an additive for high-strength alloys and an electrode material for fuel cells. However, scandium has not yet been widely used due to small production quantity and high cost thereof.
Meanwhile, a trace amount of scandium is contained in nickel oxide ore such as laterite ore and limonite ore. However, nickel oxide ore has not been industrially used as a raw material for nickel for many years because the content level of nickel in nickel oxide ore is low. Consequently, very few studies also have been conducted for industrially recovering scandium from nickel oxide ore.
However, in recent years, the High Pressure Acid Leaching (HPAL) process has been emerging for practical use, in which nickel oxide ore is introduced into a pressure vessel along with sulfuric acid, and heated at a high temperature of about 240 to 260° C. to allow solid-liquid separation into a nickel-containing leachate and a leach residue. A neutralizing agent is added to a leachate obtained from the HPAL process to separate impurties, and then a sulphidizing agent is added to recover nickel as nickel sulfide. Subsequently, this nickel sulfide is subjected to a known nickel refinement process to obtain electrolytic nickel and nickel salt compounds (see Patent Document 1).
FIG. 2 shows a flow chart for recovering metal from nickel oxide ore according to the publicly known technology. When the HPAL process is used (Steps S101 to S103 in FIG. 2), scandium contained in nickel oxide ore will be contained in a leachate along with nickel (Step S101 in FIG. 2). Subsequently, when a neutralizing agent is added to the leachate obtained from the HPAL process to give a pH of 1 or more to less than 4 to separate impurities (Step S102 in FIG. 2), and a sulphidizing agent is then added (Step S103), nickel is recovered as nickel sulfide while scandium remains in the post-sulfuration liquid after addition of the sulphidizing agent. Therefore, nickel and scandium can effectively be separated when the HPAL process is used.
Then, scandium contained in the post-sulfuration liquid can be separated from impurities such as manganese by allowing scandium to be adsorbed by a chelating resin having a functional group of iminodiacetate (Step S104 in FIG. 2). Further, it is also proposed to concentrate scandium after adsorbed by the chelating resin (Step S105 in FIG. 2). Technologies for allowing a chelating resin to adsorb scandium contained in a post-sulfuration liquid and for performing further concentration are disclosed in Patent Documents 2 to 4 and the like.
However, the scandium recovery steps as described in Steps S104 and S105 of FIG. 2 may not necessarily be provided in view of the content level, the available amount, the facility investment cost and the like. FIG. 3 shows a flow chart when the scandium recovery steps are not provided. The same symbols as in FIG. 2 are used for the HPAL process. The post-sulfuration liquid after addition of a sulphidizing agent (Step S103 in FIG. 3) is sent to a wastewater treatment step (Step S106 in FIG. 3) while maintaining a scandium content, where a neutralizing agent is added so as to obtain a pH of 4 or more to form a wastewater sediment containing scandium compounds as well as impurities such as manganese compounds. Subsequently the wastewater sediment will be disposed in landfill cites and the like. Scandium is a rare material. Therefore, there have been demands for developing a technology of recovering scandium from a wastewater sediment.
An approach for obtaining highly pure scandium oxide from scandium hydroxide or scandium carbonate has been proposed, the approach comprising: a dissolution step of dissolving scandium hydroxide or scandium carbonate in an acidic aqueous solution to obtain a scandium-containing solution; a liquid adjustment step of preparing a reduced liquid by use of a reducing agent; an adsorption step of allowing for a contact with a chelating resin to form a chelating resin having scandium adsorbed; a washing step of washing the chelating resin having scandium adsorbed with a dilute acid; a dissolution step of dissolving scandium from the chelating resin having scandium adsorbed with a strong acid to obtain a scandium-containing solution; a precipitation step of obtaining a scandium precipitate with a precipitant; and a calcining step of calcining the precipitate (see Patent Document 5).
Patent Document 1: Japanese Unexamined Patent Application, Publication No. H03-173725
Patent Document 2: Japanese Unexamined Patent Application, Publication No. H01-133920
Patent Document 3: Japanese Unexamined. Patent Application, Publication No. H09-176756
Patent Document 4: Japanese Unexamined Patent Application, Publication No. H09-194211
Patent Document 5: Japanese Unexamined Patent Application, Publication No. H09-208222